452 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



in a solution of iudophenol (2-10"^), which interferes with the metaboHsm of 

 melanin. In the rat, implantation of young optic vesicles in the anterior eye 

 chamber of adults also causes overdevelopment and thickening (Fig. 92) of the 

 outer layer (Stroeva, 1956b). Contact with mesenchyme favors pigmentation, 

 but it is certainly not the sole factor. 



These new data remain in agreement with my interpretation of 1941, mainly 

 based on the relation of the optic vesicle with the prechordal substrate and its asym- 

 metrical mode of folding. The most induced part, with the highest morphogenetic 

 potential, expands more powerfully and invaginates to form the retinal layer, 

 while the upper part, less induced, remains thinner and produces melanin, not 

 without resemblance to neural crest cells. This concept is still valid today. It 

 explains why any supplement of induction causes a retinization of the outer layer. 

 It also implies that a particular middle value is attributed to the peripheral, 

 transitional part of the retina. This is the region which has been shown by 

 Stone (1944, cf. 1948, 1953, 1957; see also Lopashov, 1955a, b) to be capable of 

 regenerating the retina of amphibians throughout life. 



Another implication is that the tissues from which the upper and outer part of 

 the optic cup are derived possess a singular potential, for it is the part of the 

 inner layer which has been the least induced, although still sufficiently to become 

 retinal. This singularity explains the phenomenon, ascertained by the many 

 cleverly performed operations of Sato {cf. 1953; see also Stone, 1953; Reyer, 

 1954a, b), namely that the upper sector of the iris exhibits the potency of regener- 

 ating the lens, if the latter has been extirpated. The bud formed in these conditions 

 by the newt is astonishingly rich in RNA (Takata, 1952). The idea that this means 

 a transfer of material (RNP?) from the retina is supported by the fact that, in 

 newt eyes, isolating the dorsal iris from the neural retina inhibits the regeneration 

 of the lens (Stone, 1958). 



The formation of the choroid has been recently restudied by Lopashov (1956), 

 who considers it to be due to a mixed contribution of the neural crest and of true 

 mesenchyme, both helped by the optic vesicle itself. 



In the prechordal part of the head, all vertebrates possess two kinds of sense 

 organs, the most elaborate of which perceives light waves, the lesser receiving 

 chemical stimuli. Similarly, the chordal part of the head is equipped with two 

 kinds of prominent sense apparatus, the most complicated reacting to "static" 

 conditions and sovmd, the other registering chemical sensation. An intricate 

 system of nerves and perception organelles for general cutaneous and other 

 perceptions completes the chordal part. 



The otocyst assumes a special rank among induced structures owing to the 

 fact that although it depends normally on a primary inductor, it is produced by 

 the epiblast after the neural plate formation. The mechanisms at play in obtaining 

 the typical inner ear are necessarily complex, and I have tried recently (1954a) 

 to review the main points of the available information. The primary inductor' 

 is part of the parachordal mesenchyme. It can perhaps be related to the mes- 



^ For the numerous experimental inductions of otocysts, see supra, passim and also Kawakami 

 (1950). 



